The present invention relates, in general, to impedance matching transformers, and more particularly to a transmission line transformer having an adjustable characteristic impedance.
In any application involving transfer of a signal from the output of one stage to the input of another stage, it is necessary that the output impedance of the first stage be matched to the input impedance of the next stage. If two stages with different impedances are coupled without the benefit of matched impedances, significant attenuation of the signal will occur. At relatively low frequencies or narrow band applications, the impedance matching function is normally accomplished by using an impedance matching transformer. The conventional impedance matching transformer generally consists of two electrical conductors wound as coils, coupled by flux linkages. Impedance matching is accomplished by means of using the proper ratio of the number of turns of each of the coils.
The frequency response of such conventional impedance matching transformers, however, precludes their use in applications involving relatively high frequencies or in applications requiring broadband response. In 1959 C. L. Ruthroff, in his paper "Some Broadband Transformers",(Proc. IRE, vol. 47, pp 1,337-1342, August 1959), made an in depth exploration of the use of transmission lines as impedance matching transformers. Following the publication of Ruthroff's treatise, transmission line transformers became widely used in high frequency impedance matching applications.
The basic transmission line transformer consisted of a twisted pair of wires wrapped around a ferrite toroid core. A peculiarity of all transmission line transformers was that, rather than the virtually infinite ratios available from a conventional transformer, there was a limited number of discrete ratios which could be realized from a transmission line transformer. For instance, a bifilar transformer as just described would yield impedance matching ratios of 1/1 or 4/1, depending upon how it was connected. A trifilar transformer would yield matching ratios of 1/1, 9/4, and 9/1.
The critical parameter for a transmission line transformer was its characteristic impedance, Z.sub.O. For optimum circuit performance, EQU Z.sub.O =(Z.sub.IN Z.sub.OUT).sup.1/2.
Characteristic impedance Z.sub.O was dependent upon a number of factors, including the wire used, the type and thickness of insulation on the wire, how tightly the wire was twisted, the permeability of the core, and how the wire was wound on the core. Other factors may have also impacted Z.sub.O. Control of these many parameters in a production environment was difficult. At relatively low frequencies, some variation from the optimum value for Z.sub.O could be tolerated. However, the need to control Z.sub.O became more acute in applications utilizing frequencies nearing the gigahertz range. Until now the only method of adjusting Z.sub.O after the transmission line transformer was manufactured was to physically move the wires on the toroid. This proved to be a very inexact method of making a precision adjustment. It also introduced the risk of inadvertently damaging either the transformer itself or possibly some adjacent circuitry.